Laminate materials with embedded heat-generating multi-compartment microcapsules

ABSTRACT

A composite material incorporates multi-compartment microcapsules that produce heat when subject to a stimulus such as a compressive force or a magnetic field. The stimulus ruptures an isolating structure within the multi-compartment microcapsule, allowing reactants within the multi-compartment microcapsule to produce heat from an exothermic reaction. In some embodiments, the composite material is a laminate used in the manufacture of multi-layer printed circuit boards (PCBs) and provides heat during the curing process of the multi-layer PCBs to ensure a consistent thermal gradient in the multi-layer product.

I. CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/247,151, filed Aug. 25, 2016. The aforementioned related patentapplication is herein incorporated by reference in its entirety.

II. FIELD OF THE DISCLOSURE

The present disclosure relates generally to laminate materials thatinclude heat-generating, multi-compartment microcapsules.

III. BACKGROUND

During conventional lamination of a multi-layer printed circuit board(PCB), a thermal gradient develops between top and bottom platens of alamination press and within the interior of the PCB stack. Depending onthe thickness of the PCB stack, the thermal gradient can result ininternal (e.g., middle) laminate layers having a different degree ofcure than the outer layers. Different degrees of cure can result indifferent glass transition temperatures, differences in the coefficientof thermal expansion of the laminate material, and other undesirableproperties that may reduce reliability and structural integrity of themulti-layer PCB.

IV. SUMMARY OF THE DISCLOSURE

According to an embodiment, a laminate material includes heat-generatingmulti-compartment microcapsules. A multi-compartment microcapsuleincludes at least two compartments. Each compartment contains areactant. The reactants produce heat when they are combined. Anisolating structure separates the compartments of the multi-compartmentmicrocapsule. The isolating structure is configured to rupture inresponse to a stimulus. In some embodiments, the multi-compartmentmicrocapsules are shell-in-shell multi-compartment microcapsules havingan inner shell contained within an outer shell. The inner shell and theouter shell contain the heat-generating reactants.

According to an illustrative embodiment, the reactants are allowed tomix upon rupture of the inner shell, while the outer shell remainsintact. The reactants produce heat while the laminate material cures. Insome embodiments, the stimulus that ruptures the isolating structure isa compressive force or a magnetic field. In an embodiment where thestimulus is a magnetic field, the inner shell of the microcapsule maycontain magnetic nanoparticles, such as magnetite (Fe₃O₄) particles,that rotate and/or vibrate at an accelerated rate upon the applicationof a magnetic field thereby rupturing the inner shell. Since the outershell of the microcapsule in this embodiment has no embedded magneticnanoparticles, the outer shell remains intact upon the application ofthe magnetic field.

According to an illustrative embodiment, a method includes applying heatto a composite material that includes a multi-compartment microcapsule.The multi-compartment microcapsule includes a first compartment thatcontains a first reactant and a second compartment that contains asecond reactant. The second compartment may be separated from the firstcompartment by an isolating structure. In such a case, the firstcompartment is configured to remain intact in response to a stimulusthat ruptures the isolating structure between the first compartment andthe second compartment. The method further includes, while applyingheat, applying a stimulus to rupture the isolating structure to generateheat due to the exothermic reaction of the first reactant and the secondreactant.

V. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of a portion of a printed circuit board(PCB) that includes a composite material sandwiched between conductivematerial, the composite material includes multi-compartmentmicrocapsules;

FIG. 2 depicts an embodiment of a portion of a PCB that includes anothertype of multi-compartment microcapsule;

FIG. 3 illustrates various embodiments of multi-compartmentmicrocapsules;

FIG. 4 depicts an embodiment of a portion of a PCB showing multiplelayers of composite material, each layer including a different amountand density of multi-compartment microcapsules;

FIG. 5 depicts the portion of the PCB illustrated in FIG. 4 showingmultiple layers of composite material after reaction of the contents ofthe multi-compartment microcapsules; and

FIG. 6 is a flow diagram showing a particular embodiment of a process tocure a self-heating composite material.

VI. DETAILED DESCRIPTION

The present disclosure relates to composite materials that incorporatemulti-compartment microcapsules dispersed within a laminate material.Composite materials include sealants, adhesives, epoxies, thermalinterface materials, coatings, paints, varnishes, and encapsulants.

The present disclosure describes a process for activatingheat-generating multi-compartment microcapsules by applying a stimulusof sufficient magnitude to rupture an isolating structure that separatescompartments of the multi-compartment microcapsules. Generally, an innerstructure ruptures while an outer shell remains intact so that thereactants and the reaction products do not contact the laminatematerial. Examples of a stimulus includes application of a compressiveforce, a magnetic field, or heat to raise the temperature of themulti-compartment microcapsules.

The present disclosure also describes a process for curing a laminatematerial with the use of embedded multi-compartment microcapsulesincluded in the laminate material. A stimulus of sufficient magnitude isapplied to the laminate material that ruptures an isolating structureseparating the compartments of the multi-compartment microcapsules. Thecontents of the compartments react exothermically thereby supplying heatwithin the laminate material itself

Including heat-generating multi-compartment microcapsules in laminatematerials provides several benefits. One benefit is a reduction incuring time. In curing, a laminate must reach a pre-determined curingtemperature. Multi-compartment microcapsules generate heat within thelaminate. Curing time is reduced because the heat within the laminate isin addition to heat provided from an external heat source. Providingheat only from an external source requires time to transfer from theexternal source into the laminate material.

Another benefit is a more even cure. Use of heat-generatingmulti-compartment microcapsules in the laminate material evens out athermal gradient that develops in the laminate as heat from an externalsource is applied to the laminate. The heat-generating microcapsulesraises the temperature from within the laminate while heat is appliedfrom the outside layers. A more uniform thermal gradient is the result.A thermal gradient is a measure of temperature differences betweentemperatures at various points in the laminate. A thermal gradient maybe monitored over time. A more uniform thermal gradient yields smallertemperature differences at any instant in time due to the internallyreleased heat from the microcapsules.

Another benefit of the additional heat from the microcapsules isdecreased viscosity of the laminate material. Viscosity is theresistance of a material to flow. If the laminate material is anadhesive, decreasing the viscosity may provide better control over abond line thickness of the adhesive applied between two substrates.

FIG. 1 depicts a cut-away view of an embodiment of a portion of aprinted circuit board 100 that includes layers of a conductive material106, such as copper, separated by a composite material 107. Thecomposite material 107 contains multi-compartment microcapsules 101. Thecomposite material 107 may include a pre-preg material, an epoxy resin,an adhesive, or a combination thereof. The composite material 107 mayalso include reinforcing fibers, such as glass fibers.

As illustrated in FIG. 1, the multi-compartment microcapsules 101 have ashell-in-shell configuration. The microcapsules 101 include an innershell 102 contained within an outer shell 103. The inner shell 102functions as an isolating structure that ruptures in response to astimulus. The inner shell 102 forms a first compartment that contains afirst reactant 104. The volume between the inner shell 102 and the outershell 103 forms a second compartment. The second compartment contains asecond reactant 105. The first and second reactants 104, 105 undergo anexothermic chemical reaction when mixed together. While the inner shell102 of the multi-compartment microcapsule 101 is intact (e.g., notruptured), the inner shell 102 prevents the first reactant 104 fromcontacting the second reactant 105. The inner shell 102 of themulti-compartment microcapsule 101 is configured to rupture whensubjected to a particular stimulus such as a physical force or amagnetic field.

In some embodiments, the multi-compartment microcapsules have acharacteristic dimension (e.g., a diameter) of 5.0 microns (μm) or less.For example, the characteristic dimension for the outer shell 103 isless than 55, 40, 30, 20, 10, 5, 2, 1, 0.5 and 0.2 μm. Thecharacteristic dimension for the inner shell 102 is less than 50, 40,30, 20, 10, 5, 2, 1, 0.5 and 0.2 μm.

In some configurations, the outer shell 103 of the multi-compartmentmicrocapsule 101 is configured to remain intact (e.g., not to rupture)when subjected to the particular stimulus. For example, the outer shell103 may be made of a same material as that of the inner shell 102 butincludes a thicker shell and thus the outer shell 103 withstands theparticular stimulus while the inner shell 102 does not. A stimulus mayinclude a mechanical force (e.g., a compressive force, compressivepressure), an electromagnetic force (e.g., application of a magneticfield), or another type of force. By applying the particular stimulus tothe multi-compartment microcapsules 101, the first and second reactants104, 105 can be mixed together to initiate the exothermic chemicalreaction.

In an embodiment, the inner shells 102 and the outer shells 103 of themulti-compartment microcapsules 101 are each formed of or include one ormore polymer layers. In the example illustrated in FIG. 1, the outershell 103 encapsulates the inner shell 102. However, in other examples,such as the examples illustrated in FIG. 3, the second reactant 105 isencapsulated in a shell that does not encapsulate the inner shell 102.

Fabrication processes vary with the materials used to form the shellsand with the reactants encapsulated by shells. A particular,non-limiting, example of a fabrication process includes in situpolymerization of urea-formaldehyde (UF) to form microcapsule shellsaround reactants. Microcapsule fabrication processes may includepreparing an aqueous solution by mixing water and ethylene maleicanhydride (EMA) copolymer, and then agitating the aqueous solution.Urea, ammonium chloride, and resorcinol may be added to the aqueoussolution.

As an example of a stimulus, a magnetic field may be used to rupture theinner shell 102 of the multi-compartment microcapsules 101. In terms offabrication for the inner shell 102, functionalized magneticnanoparticles may be added to the aqueous solution. The pH of theaqueous solution may be adjusted, e.g., by addition of sodium hydroxide(NaOH) or potassium hydroxide (KOH). The first reactant 104 may be addedto the aqueous solution to form an emulsion. The first reactant 104 maybe dissolved in a solvent for addition to the aqueous solution, or maybe added in a solid form (e.g., powder, crystal granules).

Formaldehyde may be added to the emulsion to initiate a reaction to formurea-formaldehyde (UF) microcapsules (e.g., a first polymer layerencapsulating the first reactant 104). If functionalized magneticnanoparticles are added to the aqueous solution, the functionalizedmagnetic nanoparticles may be covalently bonded to the UF microcapsules.After being formed, UF microcapsules may be cleaned, washed and dried.The UF microcapsules may be sieved to separate microcapsules that arenot within a desired size range.

In the example above, the UF microcapsules encapsulating the firstreactant 104 correspond to the inner shells 102 of FIG. 1. The outershells 103 may be formed using a similar process. For example, the UFmicrocapsules encapsulating the first reactant 104 may be added to anemulsion that includes: (1) the second reactant 105 (in solution orsolid form) and (2) reactants to form a second polymer layer (e.g., asecond UF microcapsule encapsulating the second reactant 105). In theexample illustrated in FIG. 1, the second polymer layer encapsulates thesecond reactant 105 and one or more UF microcapsules that encapsulatethe first reactant 104. In other examples, the second polymer layer mayintersect or couple to the first polymer layer, as in one or more of theexamples illustrated in FIG. 3.

In general, for a given shell diameter, thinner shells rupture morereadily than thicker shells made of a same material. The inner shell 102may be made thinner than the outer shell 103. For example, an innershell 102 having a first thickness of 1 μm is designed to rupture whenexposed to a pressure of 1.4 psi that is less than a second pressure of28.4 psi. The second pressure is a pressure at which an outer shell 103is designed to rupture. In other embodiments, a magnitude of the secondforce or pressure is a substantial multiple of a magnitude of the firstpressure. For example, the first pressure is 20 psi and the secondpressure is 7,000 psi. An outer shell 103 having a second thickness (athickness greater than the first thickness) ruptures when exposed to thesecond pressure. In the example where the inner shell 102 is made from asame material as the outer shell 103, the inner shells 102 are maderelatively thin compared to the outer shells 103. In use, the innershells 102 are ruptured while leaving the outer shells 103 intact.Rupturing the inner shells 102 allows a reaction to take place inside ofthe outer shells 103 without contaminating the material (e.g., laminatematerial, resin) with the reactants 104, 105.

In embodiments where the inner shells 102 include magnetic nanoparticlesto facilitate rupturing of the inner shells 102, the magneticnanoparticles may be covalently bound to the inner shells 102 of themulti-compartment microcapsules 101. Magnetic nanoparticles 203 areillustrated as the particles in the inner shells 202 in FIG. 2.Functionalized magnetic nanoparticles may be formed by reacting anorgano trialkoxysilane, such as aminopropyltriethoxysilane, with a mildacid (e.g., acetic acid, hydrochloric acid, formic acid) in an aqueoussolution to form a hydrolyzed silane. In addition to, or instead ofwater, a solvent, such as tetrahydrofuran, ethanol or methanol, may beused. Magnetic nanoparticles (e.g., Fe₃O₄) may be added to the solutionafter formation of the hydrolyzed silane to form functionalized magneticnanoparticles. Rupturing the inner shell 102 causes the first reactant104 to come into contact with the second reactant 105 thereby generatingheat by an exothermic reaction. Heat from the microcapsule 101 istransferred to the composite material 107 through the outer shell 103.

In accordance with some embodiments, the first reactant 104 includes orcorresponds to a reducing agent and the second reactant 105 correspondsto or includes an oxidizing agent. Alternately, the first reactant 104includes or corresponds to an oxidizing agent and the second reactant105 corresponds to or includes a reducing agent. These reactants arereadily available and react predictably and quickly to generate asignificant quantity of heat per unit weight. As an illustrativeexample, the first reactant 104 and second reactant 105 may include ironand oxygen, respectively. These materials react according to thefollowing reaction equation:

4Fe(s)+3O₂(g)===>2Fe₂O₃(s) ΔH_(rxn)=−1.65103 kJ

In an embodiment, the first reactant 104 includes iron powder, and thesecond reactant 105 includes hydrogen peroxide (H₂O₂). The firstreactant 104, the second reactant 105, or both, may also include acatalyst. A catalyst may be desired or may be necessary depending on thereactants and depending on the rate of desired heat generation from themicrocapsules 101. For the example of iron powder and hydrogen peroxide,one catalyst could be ferric nitrate which, when in contact withhydrogen peroxide, liberates oxygen to react exothermically with theiron powder. In this example, the second reactant 105 may use 1.5 molesof hydrogen peroxide per mole of iron in the first reactant 104, or 0.56grams of iron powder to 0.51 grams of hydrogen peroxide. The catalyticamount of ferric nitrate may be chosen to achieve a desired reactionrate and thereby a rate of heat generation. The reaction rate can bemeasured in units of Kilojoules per second or other suitable set ofunits.

Continuing this example, between 0.001 and 0.005 gram equivalents offerric nitrate per liter of hydrogen peroxide results in a reaction rateproducing heat at between 100 and 500 kJ/s. In some embodiments, othermetals may be used in lieu of iron or in addition to iron. For example,magnesium or a magnesium-iron alloy may be used. In some embodiments, nocatalyst is used where a catalyst is unnecessary either for the reactionto occur or to speed the rate of reaction. In this example, anothercatalyst could be used in lieu of, or in addition to, ferric nitrate,such as sodium iodide (NaI).

In the example illustrated in FIG. 1, the multi-compartmentmicrocapsules 101 each include two reactants. However, in someembodiments, the multi-compartment microcapsules 101 each include morethan two reactants so as to provide for a desired exothermic reactionand desired heat generation characteristics for the microcapsules 101.In such embodiments, the plurality of reactants remain separated in aplurality of compartments in the multi-compartment microcapsules 101until the exothermic reaction is initiated by opening the compartmentsand allowing reactants to mix. For example, a multi-compartmentmicrocapsule 101 includes a first compartment, a second compartment, anda third compartment, and so forth, and each compartment contains adifferent reactant. In some embodiments, the compartments may be in ashell-in-shell arrangement (e.g., in a nested arrangement), asillustrated in FIG. 1. Alternatively, a single outer shell mayencapsulate each of the compartments, and the reactants may be separatedby a plurality of inner membranes or barriers.

Production of multi-layer printed circuit boards provides anillustrative example of the benefits of use of the multi-compartmentmicrocapsule 101. Multi-layer printed circuit boards may be produced byforming conductive layers (generally defining portions of circuits) on asubstrate of a pre-preg material or a core laminate material. Thepre-preg material may include fibers impregnated with an epoxy or otherresin. An example of a pre-preg material used in the production ofprinted circuit boards is FR-4, which is made of woven fiberglass and aB-stage epoxy. Other common pre-preg materials include FR-5, FR-6, G-10,and G-11.

A printed circuit board may include several layers of pre-preg material,which may be cured via application of heat. Optionally, pressure may beapplied contemporaneously with application of heat. To illustrate, thelayers of pre-preg material may be placed between heated platens. Theheated platens apply heat and pressure to the layers of pre-pregmaterial to facilitate curing of the epoxy or other resin. In thisarrangement, layers of pre-preg material adjacent to the platens usuallyare heated to a higher temperature than inner layers (e.g., middlelayers of a stack of pre-preg materials), resulting in uneven heating ofthe various layers of the pre-preg material. The uneven heating canresult in uneven curing of the epoxy or other resin between the variouslayers. Layers adjacent to the platen that receive more heat, may havehigher levels of curing (e.g., more cross-linking, longer polymerchains) than layers that are not adjacent to the platen (e.g., middlelayers).

The cure level of a layer may be correlated with one or more materialproperties or with one or more electrical properties. For example,layers that have higher levels of curing may have higher glasstransition temperatures than layers with lower levels of curing. Asanother example, a coefficient of thermal expansion (CTE) of each layermay be correlated with the level of curing of the layer. As yet anotherexample, a dielectric constant of the layers may be correlated withlevel of curing. Uneven heating can be avoided by using themulti-compartment microcapsules 101. Accordingly, undesired propertiesin the PCB layers are reduced or avoided by incorporating and usingmulti-compartment microcapsules 101 to generate heat during the curingprocess.

Referring again to FIG. 1, the multi-compartment microcapsules 101 areincorporated into the composite material 107. The composite material 107is used to form the printed circuit board (PCB) 100. The compositematerial 107 may include or correspond to one or more layers of apre-preg material. Only a single layer is shown for sake of simplicityof illustration in FIG. 1. When the layers of the pre-preg material arecured using heated platens, application of a stimulus (e.g., a force orpressure applied by the platens or application of a magnetic field)causes the inner shell 102 of at least some of the multi-compartmentmicrocapsules 101 to rupture. The first and second reactants 104, 105 ofthe multi-compartment microcapsules 101 mix and react exothermically andthereby generate heat. Heat from the exothermic reaction passes to theouter shell 103 of the multi-compartment microcapsule 101 and the heatis conducted into the composite material 107. The heat helps maintain amore uniform temperature profile in the composite material 107, whichpromotes a more uniform cure of the composite material 107.

In some implementations, one layer of the pre-preg material may have adifferent amount or different concentration of the multi-compartmentmicrocapsules 101 as compared to another layer of the pre-preg material.For example, layers of the pre-preg material that are closest to theplatens during curing may have a lower concentration of themulti-compartment microcapsules 101 because these layers reach a highertemperature than inner layers during heating applied from the outsidethrough heated platens. An example of multiple layers of pre-pregmaterial is illustrated in FIG. 4.

Layers of the pre-preg material that are internal (e.g., further fromthe platens during curing) may be provided with a higher concentrationof the multi-compartment microcapsules 101 in order to offset thetemperature profile over time. That is, the inner layers are heated withmicrocapsules 101 while the outer layers are heated predominantly withapplication of the platens. In this example, the multi-compartmentmicrocapsules 101 in the internal layers generate more heat per unitmass of pre-preg material because the microcapsules 101 are found in alarger amount or in a higher concentration than in the external layers.Thus, increased heat generation from the exothermic reaction of thefirst and second reactants 104, 105 helps to offset or nearly offsets athermal or temperature gradient at any instant of time. A conventionalthermal gradient is undesirable and is due to the uneven application ofheat to the external layers by hot platens.

In some implementations, multi-compartment microcapsules 101 havingdifferent rupture characteristics or different exothermic reactioncharacteristics may be used. For example, the curing process may beperformed in sequential stages in a heated press in which appliedpressure and applied temperature may vary in each stage. In thisexample, a first set of the multi-compartment microcapsules 101 has afirst rupture characteristic, and a second set of the multi-compartmentmicrocapsules 101 has a second, different rupture characteristic. Boththe first set and second set of microcapsules are included in the samelayer or layers. The first rupture characteristic causes the innershells 102 of the first set of multi-compartment microcapsules 101 torupture during a first stage of the curing process. The second rupturecharacteristic causes the inner shells 102 of the second set ofmulti-compartment microcapsules 101 to rupture during a second stage ofthe curing process. In this example, both stages are necessary forcuring.

To illustrate, the inner shells 102 of the first set ofmulti-compartment microcapsules 101 have a first wall thickness, and theinner shells 102 of the second set of multi-compartment microcapsules101 have a second wall thickness different from the first wallthickness. The first wall thickness is configured or selected to ruptureat a first pressure, and the second wall thickness is configured orselected to rupture at a second pressure greater than the firstpressure. The first pressure may correspond to a pressure applied by theheated press during the first stage of the curing process. The secondpressure may correspond to the pressure applied by the heated pressduring the second stage of the curing process. Thus, by using differentwall thicknesses of the inner shell 102—and thereby forming two types ofmulti-compartment microcapsules 101—the pressure at which the innershell 102 ruptures can be chosen so that the exothermic reaction betweenthe first reactant 104 and second reactant 105 is triggered during aspecific stage of various sequential stages of the curing process.

In this example, the compressive force or pressure applied to the PCB100 by the heated press may be within a first range that is typical formanufacture of PCBs. The inner shell 102 of the multi-compartmentmicrocapsule 101 is configured to sustain, without rupturing, a pressureless than a lower bound of the first range but ruptures at pressureswithin the first range of pressures. In contrast, the outer shell 103 ofthe multi-compartment microcapsule 101 is configured to sustain, withoutrupturing, a pressure less than an upper bound of the first range. Inone embodiment, the first range is from about 450 psi to about 600 psi.

Multi-compartment microcapsules 101 may be designed to work via one ormore various rupture mechanisms. To illustrate, a first set ofmulti-compartment microcapsules 101 may be configured to rupture whensubjected to a magnetic field, while a second set of themulti-compartment microcapsules 101 may be configured to rupture whensubjected to a mechanical force (e.g., a compressive force, compressivepressure).

FIG. 2 depicts a cut-away view of another embodiment of a portion of aprinted circuit board (PCB) 200. In FIG. 2, the multi-compartmentmicrocapsules 201 have a shell-in-shell configuration with an innershell 202 contained within an outer shell 103. In FIG. 2, the innershell 202 is configured to rupture in response to application of amagnetic field. For example, a plurality of magnetic nanoparticles 203are incorporated into the inner shell 202 of the multi-compartmentmicrocapsule 201. The magnetic nanoparticles 203 may include, forexample, magnetite (Fe₃O₄) or cobalt ferrite (CoFe₂O₄). The magneticnanoparticles 203 may have a characteristic dimension (such as anaverage or maximum diameter) in a range from about 6 nanometers (nm) toabout 25 nm.

Application of a magnetic field of sufficient strength ruptures theinner shell 202. The outer shell 103 of the multi-compartmentmicrocapsule 201 does not include the magnetic nanoparticles 203 (orincludes fewer of the magnetic nanoparticles 203). In an alternativeembodiment, the outer shell 103 does not include the magneticnanoparticles 203 and is thicker than the inner shell 202. Accordingly,the outer shell 103 is configured to remain intact when themulti-compartment microcapsule 201 is exposed to a magnetic field. In aparticular example, application of a sufficiently strong time-varyingmagnetic field to the PCB 200 causes the magnetic nanoparticles 203 torotate and/or vibrate resulting in rupture of the inner shell 202. Insome examples, the time-varying magnetic field has a frequency in arange from about 50 kHz to about 100 kHz and a strength in a range fromabout 2 kA/m to about 3 kA/m. Other ranges and strengths are possibleand are matched to one or more characteristics of the nanoparticles 203,the inner shell 202, or the combination of nanoparticles 203 and innershell 202.

Rupturing the inner shell 202 causes the first reactant 104 to come intocontact with the second reactant 105. Mixing of the reactants 104, 105allows the exothermic reaction to proceed and thereby generate heat forcuring of the composite material 107. In a particular embodiment, thereaction is exothermic and generates heat, as described with referenceto FIG. 1. Other types of useful reactions are possible. In the casedescribed above, a thermal gradient formed in the PCB 200 can be changedto overcome side effects of application of heat from a heat platenpressed to the outside surfaces of the PCB 200. That is, a conventionalthermal gradient can be counteracted with use of multi-compartmentmicrocapsules 201 and triggering of an available exothermic reactionoccurring inside of the composite material 107.

FIG. 3 illustrates several cross-sectional views of other embodiments ofmulti-compartment microcapsules. A first multi-compartment microcapsule301 of FIG. 3 illustrates an example of a multi-compartment microcapsulethat includes more than two reactants. The first multi-compartmentmicrocapsule 301 has a shell-in-shell architecture with two inner shells302, 303, each with respective contents 304, 305. The first content 304includes a first reactant, and the second content 305 includes a secondreactant. In other examples, the first and second contents 304, 305 mayboth include the first reactant. A remaining volume inside an outershell of the first multi-compartment microcapsule 301 may include athird reactant 306. The inner shells 302, 303, when unruptured, isolatethe contents 304, 305 from each other and from the third reactant 306.In some embodiments, the respective inner shells 302, 303 may beconfigured to rupture responsive to different stimuli. For example, thefirst inner shell 302 may be configured to rupture responsive to a firstforce (which may be mechanical or magnetic), and the second inner shell303 may be configured to rupture responsive to a second force (which maybe mechanical or magnetic). In this example, the first force may be ofthe same type (e.g., mechanical or magnetic) as the second compressiveforce, or the first and second forces may be of different types (e.g.,the first may be mechanical and the second may be magnetic).

A second multi-compartment microcapsule 311 of FIG. 3 includes an outershell 312 and a set of inner shells 313 within the outer shell 312. Eachof the inner shells 313 contains a reactant 314. The thickness of theinner shells 313 may be thinner than the thickness of the outer shell312. When one or more of the inner shells 313 rupture, the reactant 314may be exposed to a solid inner core 315 which is a second reactant.Alternatively, the inner core 315 may be liquid, a gel or a colloidsuspension.

A third multi-compartment microcapsule 321 of FIG. 3 includes an outershell 322 which encloses two inner shells 323, 325. A first inner shell323 contains a first reactant, and a second inner shell 325 contains asecond reactant. A space between the outer shell 322 and the innershells 323, 325 of the third multi-compartment microcapsule 321 does notinclude a reactant while the inner shells 323, 325 are intact (e.g.,unruptured). Both of the inner shells 323, 325 rupture in response to astimulus that, in turn, allows the first reactant 324 to mix with, andexothermically react with, the second reactant 326. The inner shells323, 325 may rupture responsive to the same stimulus, or in response todifferent stimuli. To illustrate, both of the inner shells 323, 325 mayrupture when a sufficient compressive force is applied to the thirdmulti-compartment microcapsule 321. As another illustrative example, thefirst inner shell 323 may rupture when a magnetic field is applied tothe third multi-compartment microcapsule 321, and the second inner shell325 may rupture when a sufficient compressive force is applied to thethird multi-compartment microcapsule 321.

A fourth multi-compartment microcapsule 331 of FIG. 3 includes an outershell 332 and an inner membrane 333. The membrane 333 may be configuredto rupture responsive to a stimulus. When the membrane 333 ruptures,contents of a first compartment 334 are allowed to mix with contents ofa second compartment 335. The contents of the first compartment 334includes a first reactant 336, and the contents of the secondcompartment 335 includes a second reactant 337.

FIG. 3 illustrates some variations of embodiments of multi-reactantmicrocapsules. According to some embodiments, a sequence of stimuli isneeded to mix reactants. By sequential application of specific types ofstimuli, a sequence of reactions may be performed in order to sustainheat generation over time or to alter the progression of a heat profileover time or to meet some other design goal such as to halt a reactionat a certain point or when a material reaches a certain temperature.Alternatively, sequential application of different stimuli maycorrespond to taking steps needed to initiate a single reactionrequiring multiple reactants to be added in a particular order.

In the case of a compressive force, there are various mechanisms forrupturing a first or inner compartment and not a second or outercompartment of a multi-compartment microcapsule. For example, an innercapsule may be made of a relatively brittle material. The inner capsuleor shell is broken by application of an external pressure. The externalpressure causes small, uneven sheer forces in the inner shell wallsufficient to rupture it and not rupture an outer shell wall. The outershell wall remains intact for one or more reasons. The inner shell andouter shell can be made of a same or different material but can be madeof different thicknesses. Sheer forces due to application of acompressive force to the laminate are not sufficient to break a thickerouter shell wall. Alternatively, an inner shell may be made of a brittlematerial and the outer shell wall may be made of a resilient material.The inner shell wall ruptures when exposed to a first pressure due touneven sheer forces developed in the inner brittle shell wall. The outershell is able to flex and resist the same uneven sheer forces.

Another mechanism to rupture an inner shell is the application of heatwhere the inner shell undergoes a phase change from solid to liquid at aparticular temperature or sufficiently softens that perforations ortears appear in the inner shell wall. For this mechanism, an example ofa stimulus is application of microwave energy to the microcapsules 101.The inner shell 102 reaches a softening point sooner than a softeningpoint of the outer shell 103. According to one example, the inner shellis made of a material with a lower softening temperature than thematerial used for the outer shell. The inner shell material may be madeof a cured resin that contains a thermoplastic resin with a low thermalresistance.

In some implementations, the microcapsules are designed to be activatedby a combination of stimuli, either in combination or in sequence. Forexample, a compressive force is applied simultaneously with a magneticfield. The magnetic field lowers the rupture strength of the inner shell102 such that a small amount of compressive force completes theformation of openings in the inner shell wall. As another example, heatand a compressive force may be simultaneously or sequentially applied tothe microcapsules 101. The heat raises the temperature of the innershell 102 to a point where application of a compressive force is able torupture the inner shell 102 while allowing the outer shell 103 to remainintact.

FIG. 4 depicts a multi-layer printed circuit board (PCB) 400 includinglayers of conductive material 106 separated by respective layers ofcomposite material such as composite material layers 401, 402, 403, 404,405 according to an illustrative example. The multi-layer PCB 400 is anexample of an article of manufacture. Each of the composite materiallayers 401-405 contains multi-compartment microcapsules 101. Each of thecomposite material layers 401-405 corresponds to a layer of thecomposite material 107 of FIG. 1 or FIG. 2. The multi-layer PCB 400 mayalso include outer layers of a conductive material 406.

In FIG. 4, the number or concentration of multi-compartmentmicrocapsules 101 included in each composite material layer 401, 402,403, 404, 405 varies according to the location of the layer in themulti-layer printed circuit board 400 in a vertical dimension.Specifically, the outer layers 401, 405 have a lower concentration oramount of multi-compartment microcapsules 101 than each of therespective inner layers 402-404. The inner-most layer 403 has a higherconcentration or amount of multi-compartment microcapsules 101 thaninner layers 402, 404.

During a curing process, outermost composite material layers 401, 405proximate to the outermost layers of conductive material 406 may receivemore heat from an external source (e.g., a heated platen) than does aninnermost composite material layer 403 of the multi-layer PCB 400because the heat from the external source is applied from the top andbottom. That is, the outermost composite material layers 401, 405experience a higher temperature than the other layers 402-404. Thus, asillustrated in FIG. 4, the innermost composite material layer 403includes a greater number (e.g., greater concentration, greater overallamount) of multi-compartment microcapsules 101 than the compositematerial layers 401, 402, 404, 405 closer to the outermost layers ofconductive material 406 in the multi-layer PCB 400 (with thickness ofcomposite material layer held constant).

When activated (e.g., by rupturing the inner shell of each), the greateramount of multi-compartment microcapsules 101 of the innermost compositematerial layer 403 generate more heat per unit of composite materialthan the lesser amounts of multi-compartment microcapsules 101 of theother composite material layers 401, 402, 404, 405. By generating moreheat via chemical reaction of the multi-compartment microcapsules 101, atemperature profile of the printed circuit board 400 during the curingprocess may be made more uniform as observed from top to bottom in thePCB 400. The increased heat generated by the multi-compartmentmicrocapsules 101 of the innermost composite material layer 403compensates for the conventional thermal gradient that would normallydevelop due to application of heat at the outer layers of conductivematerial 406. Improved curing results from the improved temperature orthermal gradient.

In some implementations, the amount or concentration ofmulti-compartment microcapsules 101 in one or more of the other layers(e.g., composite material layers 401, 402, 404, 405) may be related to adistance of the layer from the outer layers 406 where heat is applied.The specific amount or concentration of the multi-compartmentmicrocapsules 101 to be added to each layer may be determined as afunction of certain factors such as: the heat generated by each of themulti-compartment microcapsule 101, thickness of each layer, the amountof pressure required to rupture the inner shell 102 of amulti-compartment microcapsule 101, and the temperature profile or thethermal gradient in the multi-layer PCB 400.

Additionally, or in the alternative to varying the amount orconcentration of the multi-compartment microcapsule 101 based on thedistance of each layer from an external heat source, different types ofmulti-compartment microcapsule 101 may be used for different layers. Forexample, the multi-compartment microcapsule 101 of the innermostcomposite material layer 403 may include a first set of reactants (or afirst quantity of reactants) configured to generate a first quantity ofheat upon reaction. In this example, one or more of the compositematerial layers 401, 402, 404, 405 may include multi-compartmentmicrocapsule 101 that include a second and different set of reactantsconfigured to generate a different quantity of heat upon reaction.

Alternatively, according to another embodiment, the multi-compartmentmicrocapsules 101 include a different quantity of the same reactants ora different concentration of same reactants in a same or similar volumeof microcapsule 101. The multi-compartment microcapsules 101 of theinnermost composite material layer 403 are configured to generate moreheat (per multi-compartment microcapsule 101) than the multi-compartmentmicrocapsules 101 of one or more of the composite material layers 401,402, 404, 405. The microcapsules 101 of the composite material layers402, 404 generate more heat than the outermost layers 401, 405.

FIG. 5 depicts the portion of the PCB illustrated in FIG. 4 showingmultiple layers of composite material and microcapsules 501 according toone embodiment of a cured multi-layer PCB 500. The microcapsules 501 arespent multi-compartment microcapsules because the reactants (e.g., thereactants 104, 105 of FIG. 1) have reacted with each other. In thisembodiment, the outer shell (e.g., the outer shell 103) remains intact.The inner shell has been ruptured and pieces 502 of the former innershell are inside the spent microcapsules 501. Reaction end products 504are also inside the microcapsules 501. The PCB 500 may also include someunspent multi-compartment microcapsules 101. Alternatively, all of themicrocapsules 501 in the PCB 500 are spent.

The cured multi-layer PCB 500 includes layers of conductive material 106separated by respective layers of composite material, such as compositematerial layers 401, 402, 403, 404, 405. The multi-layer PCB 500 is anexample of an article of manufacture. Each of the composite materiallayers 401-405 contains spent microcapsules 501, 503 and may include oneor more unspent multi-compartment microcapsules 101. Each of thecomposite material layers 401-405 corresponds to a layer of thecomposite material 107 of FIG. 1 or FIG. 2. The multi-layer PCB 500 mayalso include layers of a conductive material 406.

In FIG. 5, the number or concentration of spent microcapsules 501included in each composite material layer 401, 402, 403, 404, 405 variesaccording to the location of the layer in the multi-layer printedcircuit board 500 in a vertical dimension. The outer layers 401, 405have a lower concentration or amount of microcapsules 501 than each ofthe respective inner layers 402-404. The inner-most layer 403 has ahigher concentration or amount of spent microcapsules 501 than innerlayers 402, 404.

When a cured multi-layer PCB 500 is inspected, the presence of the spentmicrocapsules 501 having reaction end products 504 is one indicationthat multi-compartment microcapsules 101 were present and were activatedduring curing of the PCB 500. The presence of pieces of inner shell 502and the reaction end products 504 inside of a spent microcapsule 501 isanother indication that multi-compartment microcapsules 101 were presentand were activated. For an embodiment of a microcapsule having anisolating structure (such as membrane 333 illustrated in FIG. 3),instead of pieces of inner shell 502, the isolating structure orportions thereof may be found in a spent microcapsule 501.

According to an alternative embodiment, during curing, some of the spentmicrocapsules 501 deform and appear in the cured multi-layer PCB 500 asdeformed microcapsules 503. That is, when exposed to pressure and heat,such as the heat generated from an exothermic reaction in themicrocapsules 101, the outer shells of spent microcapsules 501 softenedand deformed, but remained intact as illustrated in FIG. 5 as deformedmicrocapsules 503.

According to yet another alternative embodiment, the outer shell of aspent microcapsule 501 does not remain intact during a curing process.Instead, pieces of inner shell 502, pieces of outer shell, and reactionend products 504 are found in pockets in one or more layers of compositematerial 401-405.

FIG. 6 is a flow diagram showing a particular embodiment of a method 600to cure a composite material. The method 600 includes, at 601, forming astack including multiple layers of a composite material. For example,the stack may include or correspond to the multi-layer printed circuitboard 400 of FIG. 4. The PCB 400 includes layers of conductive material106 and composite material layers 401-405. In other examples, the stackmay include more layers, fewer layers, different types of layers, or adifferent arrangement of layers. The layers may be uniform ornon-uniform in thickness along its width or length, and each layer maybe of a different thickness as compared to other layers. One or more ofthe layers of composite material may include a pre-preg material, suchas a resin embedded in or coupled to fiberglass. Additionally, one ormore of the layers of composite material may include multi-compartmentmicrocapsules, such as the multi-compartment microcapsules 101, themulti-compartment microcapsules 201, the multi-compartment microcapsules301, 311, 321, 331, or a combination thereof.

The method 600 also includes, at 602, applying heat to one to morelayers of the stack.

For example, the stack may be placed between platens of a press. Thepress may apply heat and pressure to the stack to effect curing of thecomposite material. In this example, generally, the heat is applied viaconduction from the top platen to a top layer of the stack, from thebottom platen to a bottom layer of the stack, or both. Thus, atemperature gradient in the stack may be such that inner layers of thestack are cooler than outer layers of the stack at any given instant oftime.

The method 600 also includes, at 603, applying a stimulus to one or morelayers of the stack to rupture one or more heat generating microcapsulesof the one or more layers. For example, the stimulus may include orcorrespond to the pressure applied by the press. To illustrate, thepress may apply a force to the stack that is greater than or equal to arupture threshold of a barrier layer of one or more of themicrocapsules. The barrier layer may separate reactants within themicrocapsules. Rupturing the barrier layer allows the reactants to mixand undergo an exothermic chemical reaction. As another example, thestimulus may include or correspond to a magnetic field. To illustrate,the press may apply a time varying magnetic field that is sufficient tocause magnetic particles bonded to the barrier layers of one or more ofthe microcapsules to rupture. The barrier layers may include orcorrespond to the inner shells 102 of FIG. 1, the inner shells 202 ofFIG. 2, one or more of the inner shells 302, 303, 313, 323, 325 of FIG.3, the inner membrane 333 of FIG. 3, or a combination thereof.

Heat released as a result of the exothermic chemical reaction mayincrease a temperature of one or more layers of the stack. Toillustrate, the temperature of one or more inner layers of the stack maybe increased in order to decrease a temperature gradient of the stack.By decreasing the temperature gradient of the stack, the heat of theexothermic chemical reaction may facilitate uniform curing of thecomposite material or increased uniformity of curing of the compositematerial.

In some embodiments, the method 600 may include application of stimuliin stages. For example, after applying the stimulus at 603, a differentor additional stimulus may be applied to rupture additional barrierlayers of other microcapsules. Thus, heat generated due to exothermicreaction of reactants in the microcapsules of the stack may be releasedin stages to control or regulate a temperature within the stack overtime to thereby shape a temperature gradient of the stack over time tofacilitate uniform curing of the composite material of the stack.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the disclosedembodiments. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thescope of the disclosure. Thus, the present disclosure is not intended tobe limited to the embodiments shown herein but is to be accorded thewidest scope possible consistent with the principles and features asdefined by the following claims.

1. An apparatus, comprising: a cured composite material, comprising: afirst layer; and a second layer coupled to the first layer, wherein thesecond layer includes microcapsules, and wherein each microcapsuleincludes an outer shell containing a reaction end product and a portionof an isolating structure.
 2. The apparatus of claim 1, wherein thefirst layer includes microcapsules, and wherein the microcapsules in thefirst layer are included at a higher density per unit volume of curedcomposite material than a density of microcapsules included in thesecond layer.
 3. The apparatus of claim 1, wherein the cured compositematerial includes one or more of an adhesive and an epoxy resin.
 4. Theapparatus of claim 1, wherein at least some of the microcapsules of thesecond layer include a catalyst for a reaction yielding the reaction endproduct.
 5. The apparatus of claim 1, wherein the isolating structureincludes a piece of an inner shell, and wherein the isolating structureis made of a same material as the outer shell.
 6. The apparatus of claim1, further comprising a conductive layer coupled to the first layer, thesecond layer, or a combination thereof.
 7. The apparatus of claim 1,wherein the microcapsules comprise: a first shell encapsulating a firstreactant; and a second shell encapsulating a second reactant, whereinthe first shell encapsulates the second shell, and wherein the firstshell is configured to remain intact in response to a stimulus strongenough to rupture the second shell.
 8. The apparatus of claim 7, whereinthe first reactant comprises a metal, a reducing agent, or a combinationthereof.
 9. The apparatus of claim 7, wherein the first reactantcomprises a catalyst, ferric nitrate, sodium iodide, or a combinationthereof.
 10. The apparatus of claim 7, wherein the second reactantcomprises an oxidizing agent, water, hydrogen peroxide, or a combinationthereof.
 11. The apparatus of claim 7, wherein the second shell of themicrocapsules further includes magnetic nanoparticles embedded therein.12. The apparatus of claim 11, wherein the magnetic nanoparticlescomprise magnetite, cobalt ferrite, or a combination thereof.
 13. Anapparatus, comprising: a cured composite material, comprising: a firstlayer; and a second layer coupled to the first layer, wherein the secondlayer includes microcapsules, wherein each microcapsule includes anouter shell containing a reaction end product and a portion of anisolating structure, and wherein the microcapsules comprise: a firstshell encapsulating a first reactant; and a second shell encapsulating asecond reactant, wherein the first shell encapsulates the second shell,and wherein the first shell is configured to remain intact in responseto a stimulus strong enough to rupture the second shell.
 14. Theapparatus of claim 13, wherein the cured composite material includes oneor more of an adhesive and an epoxy resin.
 15. The apparatus of claim13, wherein at least some of the microcapsules of the second layerinclude a catalyst for a reaction yielding the reaction end product. 16.The apparatus of claim 13, wherein the first reactant comprises a metal,a reducing agent, or a combination thereof.
 17. The apparatus of claim13, wherein the first reactant comprises a catalyst, ferric nitrate,sodium iodide, or a combination thereof.
 18. The apparatus of claim 13,wherein the second reactant comprises an oxidizing agent, water,hydrogen peroxide, or a combination thereof.
 19. The apparatus of claim13, wherein the second shell of the microcapsules further includesmagnetic nanoparticles embedded therein.
 20. An apparatus, comprising: acured composite material, comprising: a first layer; and a second layercoupled to the first layer, wherein the second layer includesmicrocapsules, wherein each microcapsule includes an outer shellcontaining a reaction end product and a portion of an isolatingstructure, and wherein the microcapsules comprise: a first shellencapsulating a first reactant, the first reactant comprising a metal, areducing agent, or a combination thereof; and a second shellencapsulating a second reactant, wherein the first shell encapsulatesthe second shell, and wherein the first shell is configured to remainintact in response to a stimulus strong enough to rupture the secondshell.